[elliotyang]

There re how many resonance structure for ClO4-? i could only draw three.

this is the answers given by my tutor. But i do not understand why the second structure is accepted as one of the O atom has only 6 valence, which does not fulfil the octet rule.

[Borek]

Typo if you ask me.

[AWK]

Second structure
move minus sign with two electrons to the right oxygen atom

[Mitch]

I also agree your tutor made a typo, good catch.

[receipt]

Sorry to *Ignore me, I am impatient* this topic, but for this .. perchlorate ion, are more resonance structures possible if the electrons in the double bond were bumped onto an oxygen, and the chlorine would thus have a formal charge of +1 (and more as you do this with all the oxygens) and that particular oxygen with a formal charge of -1? How about, bumping the electrons from the double bond to the chlorine, making it -1 and the oxygen +1?

This has been confusing me because in some cases you do this and in some cases you dont. For example the resonance for formaldehyde. The electrons in the double bond gets bumped to the oxygen, giving the oxygen a -1 formal charge, and the carbon a +1 formal charge. But in something like the carbonate ion, this does not happen, and same with the situation here for perchlorate ion.